Light emitting devices may include, for example, incandescent bulbs, fluorescent lights, and semiconductor light emitting devices, such as light emitting diodes (“LEDs”). LEDs may include a series of semiconductor layers that may be epitaxially grown on a substrate such as, for example, a sapphire, silicon, silicon carbide, gallium nitride, or gallium arsenide substrate. One or more semiconductor p-n junctions are formed in these epitaxial layers. When a sufficient voltage is applied across the p-n junction, electrons in the n-type semiconductor layers and holes in the p-type semiconductor layers flow toward the p-n junction. As the electrons and holes flow toward each other, the electrons will “collide” with corresponding holes and recombine, such that a photon of light may be emitted. The wavelength distribution of the light generated by an LED generally depends on the semiconductor materials used and the structure of the thin epitaxial layers that make up the “active region” of the device, which is where the electron-hole recombination occurs.
LEDs are nearly monochromatic light sources that appear to emit light having a single color. The spectral power distribution of the light emitted by LEDs may be centered about a “peak” wavelength, which is the wavelength where the spectral power distribution or “emission spectrum” of the LED reaches its maximum as detected by a photodetector. The width of the spectral power distribution of LEDs is typically between about 10 nm and 30 nm, where the width may be measured at half of the maximum illumination on each side of the emission peak (this width may be referred to as the full width at half maximum or “FWHM” width).
Visible light may include light having many different wavelengths. The apparent color of visible light can be illustrated with reference to a two-dimensional chromaticity diagram, such as the 1931 CIE Chromaticity Diagram illustrated in FIG. 1, which provides a reference for defining colors as weighted sums of colors. As shown in FIG. 1, colors on the CIE Chromaticity Diagram are defined by x and y coordinates (i.e., chromaticity coordinates, or color points) that fall within a generally U-shaped area. Colors on or near the outside of the area are saturated colors composed of light having a single wavelength, or a very small wavelength distribution, while colors on the interior of the area are unsaturated colors that are composed of a mixture of different wavelengths. White light, which can be a mixture of many different wavelengths, is generally found near the middle of the diagram, in the region labeled 10 in FIG. 1. There are many different hues of light that may be considered “white,” as evidenced by the size of the region 10. For example, some “white” light, such as light generated by high-power sodium vapor lighting devices, may appear yellowish in color, while other “white” light, such as light generated by some fluorescent lighting devices, may appear more bluish in color.
A combination of light from light sources emitting light of first and second colors may appear to have a different color than either of the two constituent colors. The color of the combined light may depend on the wavelengths and relative intensities of the two light sources. For example, light emitted by a combination of a blue source and a red source may appear purple or magenta to an observer. Similarly, light emitted by a combination of a blue source and a yellow source may appear white to an observer. Each point in the graph of FIG. 1 is referred to as the “color point” of a light source that emits a light having that color. As shown in FIG. 1 a locus of color points that is referred to as the “black-body” locus 15 exists which corresponds to the location of color points of light emitted by a black-body radiator that is heated to various temperatures. The black-body locus 15 is also referred to as the “Planckian” locus because the chromaticity coordinates (i.e., color points) that lie along the black-body locus obey Planck's equation: E(λ)=Aλ−5/(eB/T−1), where E is the emission intensity, X is the emission wavelength, T is the color temperature of the black-body and A and B are constants. Color coordinates that lie on or near the black-body locus 15 yield pleasing white light to a human observer.
As a heated object becomes incandescent, it first glows reddish, then yellowish, and finally bluish with increasing temperature. This occurs because the wavelength associated with the peak radiation of the black-body radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants that produce light which is on or near the black-body locus 15 can thus be described in terms of their correlated color temperature (CCT). As used herein, the term “white light” refers to light that is perceived as white, is within 7 MacAdam ellipses of the black-body locus on a 1931 CIE chromaticity diagram, and has a CCT ranging from 2000K to 10,000K. White light with a CCT of 4000K may appear yellowish in color, while white light with a CCT of 8000K or more may appear more bluish in color, and may be referred to as “cool” white light. “Warm” white light may be used to describe white light with a CCT of between about 2500K and 4500K, which is more reddish or yellowish in color.